This invention relates to rechargeable cells having a MnO2 positive electrode material, a potassium hydroxide electrolyte and zinc as the negative electrode material. The usual embodiment may be the typical xe2x80x9cbobbinxe2x80x9d type cylindrical cell, however, spirally wound cells, button or coin cells, and rectangular plate cells or batteries may be provided in keeping with the present invention.
Primary and rechargeable manganesedioxide-based alkaline cells are well known and include a positive electrode having manganese dioxide (MnO2) as an active material, a negative electrode utilizing zinc as the active material, an aqueous solution of potassium hydroxide as electrolyte, and a separator between the positive and negative electrode.
To overcome recharge problems of the MnO2 in the positive electrode, cells were developed in which the discharge capacity of the cell was limited by imposing a zinc electrode limitation. Due to problems with the rechargeability of the MnO2 cathode, these cells experience capacity fading on deep discharge, which results in a progressing reduction of the discharge time. Capacity fade shows that the manganese dioxide electrode is not fully reversible, therefore, a loss of available capacity will be observed from one discharge/charge cycle to the next. Numerous approaches have been taken to reduce the capacity fade experienced such as employing various additives to the positive and the negative electrodes.
In this regard, reference is made to Kordesch et al in German patent number 3,337,568 issued Apr. 25, 1984. This patent describes a method for producing Electrolytic manganese dioxide that is doped with titanium. Such a titanium-doped MnO2 is particularly suitable for use in rechargeable manganese dioxide/zinc cells.
Kosaka et al in U.S. Pat. No. 5,026,617 issued Jun. 25, 1991, describes a rechargeable alkaline cell utilizing titanium dioxide additions to the separator for improved electrolyte retention, improved corrosion resistance and for preventing the formation of internal shorts from zinc dendrites.
Taucher et al, in WO 93/12551 filed Dec. 21, 1992, discloses improvements to primary and rechargeable alkaline manganese dioxide cells, containing barium compounds in an amount of 3-25% of the MnO2 positive electrode material.
Tomantschger et al, in U.S. Pat. No. 5,300,371, issued Apr. 5, 1994, teaches a rechargeable alkaline manganese dioxide cell with improved performance and cycle life containing organic binders, and silver and barium compounds added to the MnO2 positive electrode.
The present invention provides a rechargeable alkaline manganese dioxide-zinc cell having a variety of additives in the positive electrode. The cells of the present invention have a high discharge capacity on the initial discharge and a long cycle life with reduced capacity fade.
In one embodiment, the invention provides a rechargeable electrochemical cell comprising a manganese dioxide cathode, a separator, an anode and an aqueous alkaline electrolyte, the cathode including additives for increasing the cumulative discharge capacity of the rechargeable cell over the cycle life of the cell, wherein the additives comprise:
a) a first additive selected from the group consisting of barium and strontium compounds; and,
b) a second additive selected from the group consisting of titanium, lanthanum, cerium, yttrium zinc, calcium, tin and magnesium compounds.
In a preferred embodiment, the present invention provides a cell comprising a manganese dioxide cathode including an additive of a barium-based and/or strontium-base compound in an amount of from about 1% to about 15% by weight of the positive electrode. Typically, such barium and strontium compounds may be in the form of sulfates, oxides, titanates and hydroxides. The effect of the barium and/or strontium based additive is enhanced in a synergistic fashion by further addition of 0.1% to 5% of a compound selected from titanium, lanthanum, yttrium, cerium, zinc, calcium, tin and magnesium or combinations thereof. Typical forms of such compounds include sulfates, oxides, hydroxides, and sulfides.
In a further embodiment, the manganese dioxide cathode of the cell may comprise from 0.1% to 5% of a hydrogen recombination catalyst. Such catalysts can be chosen from the group consisting of silver, silver oxides, and other known compounds of silver. Alternatively, the hydrogen recombination catalyst can include metal hydrides such as Ti2Ni.